This invention relates generally to radio frequency (RF) power amplifier systems, and more particularly to a method and apparatus for locating and suppressing intermodulation distortion (IMD) products in a multi-carrier power amplifier (MCPA) system.
Ideally, RF power amplifiers would act linearly, faithfully reproducing an amplified RF signal at their output with no distortion. Unfortunately, in practice, physical RF power amplifiers can be non-linear and add a certain amount of unwanted distortion to a signal, which distortion is realized as IMD products. These IMD products cause interference over in the normal operating frequency range of the amplifier, which may impede proper transmission and reception of RF signals. Numerous techniques have been developed to reduce IMD products from amplified RF signals, including feed forward, predistortion, and linear amplification with non-linear components (LINC).
In multi-carrier power amplifier systems, the effects of IMD products such as interference and crosstalk may be compounded as a result of the close proximity of frequency bands. Multi-carrier power amplifiers (MCPA) therefore must operate at high drive levels in order to achieve the high linearity demanded by broadband applications. Energy leakage resulting from one band spilling over into another can undesirably degrade the signal-to-noise (SNR) ratio or bit-error rate (BER) of the proximate frequency bands.
One common technique to reduce IMD to acceptable levels is feed forward correction, whereby the IMD products are manipulated so that at the final summing point the IMD products substantially cancel out. Classic feed forward amplifiers use what is conventionally known as a pilot tone to assist in the control of the phase and gain of an error amplifier in order to minimize IMD. A pilot tone is generated and injected with the RF signals at the input to simulate an artificial signal whose frequency content is known. The amplifier produces amplified signals and simulated distortion products based on the pilot tone. At the output, a pilot tone receiver detects the simulated distortion, not the actual distortion, and the amplifier is aligned based upon minimization of the simulated distortion. However, because the amplifier is not aligned in accordance with the actual distortion products, they may not be entirely cancelled or may leak into the output, creating unwanted byproducts.
Another technique is to digitize the RF signals to baseband, filter out the desired frequency components, and then analyze the remaining undesired distortion components in a digital signal processor (DSP). This technique does not require the use of a pilot tone. The energy of these distortion components is located and measured in the DSP, and the feed forward loop is adjusted until the undesired components are eliminated. In one conventional design, for example, a feed forward amplification system uses mask detection compensation on an RF signal modulated according to a known modulation format. The RF signal is amplified, producing in-band frequency components and undesired out-of-band distortion components. The amplified signal is heterodyned to baseband so as to be centered about DC. A wide passband (1.25 MHz) bandpass filter is used to eliminate the in-band frequency components. A microprocessor queries a DSP for the energy at predetermined offsets (representative of an IMD location), and control signals adjust the gain-phase network of a feed forward network in accordance with the out-of-band distortion components.
The above approach operates in an environment where signals are modulated according to a single known modulation format (CDMA). However, such an approach would not be well suited for detecting narrowband signals such as TDMA and their associated IMD products, in part due to the wide bandwidth of the filter (1.25 MHz).
RF signals can be modulated according to any number of modulation formats which are well known in the art, including, for example, TDMA, GSM, CDMA, WCDMA, QAM, and OFDM, each of which have varying bandwidths. For example, the bandwidth for a WCDMA signal is 3.84 MHz (wideband), and the bandwidth for a CDMA signal is 1.25 MHz. By contrast, a GSM signal has a bandwidth of 250 kHz, and a TDMA signal has a bandwidth of only 30 kHz (narrowband). Thus, the bandwidth of a signal, depending on its modulation format, can vary from 30 kHz to 3.84 MHz. If the signals are located in a PCS frequency band (1930 to 1990 MHz), a narrowband tuner would require too much time to tune across the 60 MHz band, and a wideband tuner would not be able to detect the individual carriers of TDMA or GSM signals or their associated IMD products. In short, there is a tradeoff between the bandwidth of a tuner and the speed with which it can identify and eliminate IMD products.
Therefore, a need exists for a tunable receiver having a dynamic range sufficient to identify and eliminate IMD products from RF signals, particularly multi-carrier signals modulated according to both wideband and narrowband modulation formats.